The invention relates to vacuum tubes, more specifically grid amplifier tubes, such as triodes and especially tetrodes.
The invention relates particularly to grid tubes operating at high frequencies (several tens to several hundreds of megahertz) and with a power output through a coupled-cavity structure. The primary application of these tubes is radiowave transmission for radio and television in bands extending up to approximately 1,000 megahertz and for powers of several kilowatts to several tens of kilowatts.
The vacuum tube and the coupled cavities on which it is mounted form two mechanically separate assemblies that are combined to make them function. The tube is the amplifier element, whose main component is hermetically sealed in a chamber that has been evacuated. The coupled cavities form a mechanical structure separate from the tube on which the tube is mounted; they are not in a vacuum chamber; they operate in the open air; they extract high-frequency energy and transmit it through waveguides or coaxial cables, for example to a transmitting antenna, amplified by the tube.
However, the operations of the tube and coupled cavities are not independent of each other. In order to function, the coupled cavities must be tuned to a working frequency.
Therefore, there is a primary resonant circuit, a secondary circuit resonating at the same frequency, and a coupling means between the two. The primary resonant circuit is not completely enclosed in the coupled-cavity structure; it is partly contained in this structure but also partly contained in the vacuum tube itself; in other words, the resonant cavity of the primary circuit includes the entire area of the vacuum tube in which the high-frequency power energy is generated, and a portion of the coupled-cavity structure.
Specifically, in a tetrode connected in a coupled-cavity output circuit, the entire space between the anode and the control grid (usually called grid G2) forms an integral part of the primary resonant circuit, and it is clearly necessary to take this part into account when defining the composition of the coupled-cavity structure.
FIG. 1 shows a typical design of a grid tube (tetrode) designed to be connected in a coupled-cavity output circuit. The tetrode is shown partially cut away. The coupled-cavity output circuit is not shown in this figure.
The tube comprises an anode 10, a hollow cylindrical copper block whose interior wall partially delimits vacuum chamber 11 o the tube. This block is classically provided with peripheral fins 12 serving as radiators to cool the tube, or the outside wall of the anode is cooled by circulating water through a suitable jacket. A contact connection on anode 14 designed to provide contact with the coupled-cavity output circuit is formed by a flange mounted at the bottom of anode block 10. The general structure of the tube is generated by revolution around the central axis.
The control grid is shown is reference G2 in FIG. 1. It can be connected to the output circuit by a control grid connection 16, here composed of a flange coaxial with the contact flange of anode 14.
There can be a potential difference of several thousand volts between the anode and the control grid. It is therefore necessary to insulate the two flanges 14 and 16 electrically from one another. This insulation is provided by a ceramic strut 18 to which the flanges of anode 14 and control grid 16 are both soldered.
Ceramic strut 18 in this example is in the shape of a cylinder coaxial with connecting flanges 14 and 16.
It provides electrical insulation between the anode and the control grid for several thousands of volts; but it must also provide a perfect vacuum seal for chamber 11; it also provides a mechanical holding function for the parts of the tube (rigid connection between anode and grid); finally, it can also act as a dielectric window to allow the high-frequency energy to pass to the output circuit.
It is not necessary to describe the rest of the tube, especially the other electrodes, which are of classic design. The invention relates to the high-frequency output of the tube; in this type of tube, the output is between the anode and the control grid.
FIG. 2 is a highly schematic representation of the same tube, but this time connected to its coupled-cavity output circuit.
The coupled-cavity assembly comprises a coaxial conducting structure having a first wall 20 in electrical contact with anode 10 and a second wall 22 in contact with the connecting flange of grid 16. This structure defines two resonant circuits separated by an electromagnetic coupling means 24. In this example, coupling means 24 is a capacitive piston, free to move between the walls of the coupled-cavity structure. The piston can be fixed if the tube is to operate on a single frequency; it is movable to allow tuning to a desired frequency, within a range of adjustment related to the distance the piston can be displaced; piston 24 in fact defines the downstream end of the first structure of the cavity.
The primary resonant circuit, designed to transmit high-frequency energy to the secondary circuit through coupling means 24, comprises not only the zone above the coupling piston between walls 20 and 22 of the coupled-cavity structure, but also the entire zone inside the tube, between the control grid and the anode, where the high-frequency fields are developed when the tube operates as an amplifier.
Resonant tuning of this primary circuit therefore involves this entire zone, which is part of the vacuum chamber of the tube but not of the coupled-cavity structure, the latter being in the open air.
The secondary resonant circuit comprises the entire zone located below coupling capacitive piston 24 between walls 20 and 22 of the structure, and extending as far as a conducting piston 26 which is free to move between the walls to adjust the resonant frequency of the circuit; capacitive coupling piston 24 defines the upstream end of the second cavity; conducting piston 26 defines the downstream end, short-circuiting the second resonant cavity.
An electromagnetic coupling means 28 is provided in the secondary circuit to tap the high-frequency energy from the secondary circuit and to transmit it to a consumer circuit, not shown, for example through waveguides or coaxial cables.
In general, the primary circuit is tuned in the quarter-wavelength mode (lambda/4); in other words, the electrical length between the tip of the anode where the strongest fields develop and the coupling piston 24 corresponds to a quarter of the wavelength at the tuning frequency. Secondary circuit tuning is in the half-wavelength mode (lambda/2) corresponding to half the length of the electrical wave between pistons 24 and 26.
The high-frequency energy passes through ceramic strut 18 constituting a high-frequency, vacuum-tight dielectric window.
But this window has a considerable surface for reasons related to the high-voltage insulation it must provide between the anode and grid. It is generally made of alumina, with a very high dielectric constant (epsilon) (nine times that of vacuum).
Therefore this ceramic has a considerable capacitance. This capacitance can reach for example 3.5 picofarads for a 10-kilowatt tube operating at about 800 megahertz, while the total output capacitance of the tube, measured between the control grid and the anode, is on the order of 13 picofarads.
The influence of this capacitance is therefore very great; it is inconvenient because the lowest possible output capacitance is desired; the Q important for the tube is its gain x passband product, and this product is practically inversely proportional to the output capacitance.
The disadvantageous influence of ceramic window 18 is apparent, when we consider that this ceramic is located near to the space (under vacuum) between grid and anode, i.e. the where there are powerful electrical fields when the tube is operated as an amplifier. The ceramic window occupies an electrical length in this space that increases directly with its dielectric constant; this increases the electrical dimensions of the resonant cavity of the primary circuit, thus reducing the maximum operating frequency.
To reduce this output capacitance, it has been suggested that the alumina window be replaced by a beryllium oxide window with a lower dielectric constant, but this material is expensive and toxic, and is therefore undesirable.